The unfortunate recent collapse of the Morandi bridge in Genoa has reminded us of the importance of the degradation of concrete and cement with time. For example, cement is attacked by sulfate ions in seawater or underground, leading to expansion, strength loss and eventually the catastrophic loss of structures such as bridges. Cement expansion is usually related to ettringite and gypsum precipitation. However, despite numerous studies and expertise, the exact degradation mechanisms remain unclear and controversial. We have thus conducted a study of this mechanism in collaboration with a private company, Bouygues, and an academic partner, IFSTTAR. It aimed to characterize the possible altered zone and to identify the mechanisms of degradation under sulfate attack. The analysis of the physical and chemical aspects of this phenomenon shows that two modes of transfer occur: transfer of sulfate ions to the cement matrix and leaching of calcium ions to the external solution. The analysis of the chemical composition of the affected material highlights the progressive consumption of portlandite at the surface, the decalcification of C-S-H and the formation of AFt from both Afm and aluminum incorporated in C-S-H.